1. Field of the Invention
The present invention relates to a semiconductor laser diode including a semiconductor layered structure of Group III nitride semiconductors.
2. Description of the Related Art
Group III-V semiconductors including nitrogen as a Group V element are generally called “Group III nitride semiconductors” and typical examples thereof include aluminum nitride (AlN), gallium nitride (GaN) and indium nitride (InN), which are generally represented by a composition formula AlxInyGa1-x-yN (0≦x≦1, 0≦y≦1 and 0≦x+y≦1).
Short-wavelength laser sources such as blue and green laser sources are utilized in the fields of high density recording in optical disks typified by DVDs, image processing, medical apparatuses and measuring apparatuses. Such a short-wavelength laser source includes a laser diode employing, for example, a GaN semiconductor.
The GaN semiconductor laser diode is produced by growing Group III nitride semiconductors on a nitride gallium (GaN) substrate having a major plane defined by a c-plane through metal-organic vapor phase epitaxy (MOVPE). More specifically, an n-type GaN contact layer, an n-type AlGaN cladding layer, an n-type GaN guide layer, an active layer (light emitting layer) a p-type GaN guide layer, a p-type AlGaN cladding layer and a p-type GaN contact layer are sequentially grown on the GaN substrate through the metal-organic vapor phase epitaxy, whereby a semiconductor layered structure including these semiconductor layers is formed. In the active layer, electrons injected from the n-type layers and positive holes injected from the p-type layers are recombined, whereby the active layer generates light. The light is confined between the n-type AlGaN cladding layer and the p-type AlGaN cladding layer, and transmitted in a transmission direction perpendicular to a stacking direction in which the aforementioned semiconductor layers are stacked in the semiconductor layered structure. The active layer has resonator end faces present at opposite ends thereof with respect to the transmission direction. The light is resonantly amplified between the pair of resonator end faces by repeated induced emission, and a part of the light is emitted as a laser beam from one of the resonator end faces.
It is a conventional practice to use AlGaN for the cladding layers and use GaN for the guide layers in the GaN semiconductor laser diode irrespective of a wavelength range.
The combinational use of the AlGaN cladding layers and the GaN guide layers is suitable for confinement of light of a wavelength (e.g., 405 nm) in a blue wavelength range. However, light of a wavelength (e.g., 532 nm) in a green wavelength range cannot be properly confined, so that the light generated by the active layer is likely to leak to the outside. This problem will be described in detail with reference to FIG. 11.
FIG. 11 shows changes in refractive index with respect to the Al content x of AlGaN. In FIG. 11, curves respectively show changes in refractive index at different wavelengths, i.e., at wavelengths of 400 nm, 450 nm, 500 nm, 550 nm and 600 nm. A curve for a wavelength of 400 nm indicates that AlxGa1-xN having an Al content of zero, i.e., GaN, has a refractive index of 2.53. Further, AlxGa1-xN having an Al content of 0.07, i.e., Al0.07GaN (which means Al0.07Ga0.93N with an Al content of 0.07 in AlxGa1-xN, hereinafter expressed by a notation of Al0.07GaN for simplicity because the composition ratio between the Group III element and the Group V element is 1:1), has a refractive index of 2.47. It is generally known that a refractive index difference Δn of not less than 0.04 (preferably not less than 0.05) observed between the guide layers and the cladding layers is sufficient for the confinement of the light. In the case of a laser diode having a light emission wavelength of about 400 nm, therefore, the combination of the GaN guide layers and the Al0.07GaN cladding layers permits proper confinement of the light in an optical waveguide.
However, the change in the refractive index with respect to an increase in the Al content is reduced as the wavelength increases. Where the wavelength is not less than 500 nm, the refractive index difference Δn is less than 0.04 for the combination of the GaN guide layers and the Al0.07GaN cladding layers. Therefore, light having a wavelength (e.g., 532 nm) in the green wavelength range cannot be sufficiently confined. Since the change in the refractive index with respect to the change in the composition of the Group III nitride semiconductor depends upon the wavelength, it is necessary to properly select the compositions of the cladding layers and the guide layers according to the light emission wavelength.
FIG. 11 indicates that an AlGaN layer differing in refractive index by a difference Δn of not less than 0.04 from GaN can be formed by increasing the Al content for a light emission wavelength of not less than 500 nm.
However, if the Al content is increased, the degree of lattice mismatch with respect to the GaN substrate is increased. Therefore, cracking is liable to occur, resulting in a lower yield. In reality, this makes it difficult to increase the refractive index difference Δn between the AlGaN cladding layers and the GaN guide layers to not less than 0.04 (more preferably not less than 0.05) for a light emission wavelength of not less than 500 nm.
In view of the foregoing, it is generally difficult to increase the refractive index difference Δn to not less than 0.04 (more preferably not less than 0.05) by using the AlGaN cladding layers and the GaN guide layers in combination for a light emission wavelength range not less than 450 nm.